[Dovecot] IMAP clients suddenly redownload folder headers

[Brent Bloxam]

We had an issue today with IMAP clients (Thunderbird, Outlook) having to 
redownload headers for folders. This issue was eerily close to a MySQL 
database structure change that shouldn't have affected any related 
queries, however we did notice an event of user CPU % not being reported 
via SNMP around the same time for the server. There are no entries in 
our dovecot error log around the outage.

We're running Dovecot 1.1.13 with MySQL being used for user and password 
queries, and Maildir for storage on a fibre channel SAN. However, it is 
possible for some stores to be on an NFS mount (supporting legacy during 
migration), which is why there is the nfs options in the below config.

I'm wondering if someone could shed light on what could cause this to 
occur. I imagine it's related to the index cache's, but the lack of any 
mention in the error logs has me puzzled.

Output of dovecot -n:

# 1.1.13: /usr/local/etc/dovecot.conf
# OS: FreeBSD 7.0-RELEASE i386
base_dir: /var/run/dovecot
log_path: /var/log/dovecot-error.log
info_log_path: /var/log/dovecot-info.log
protocols: imap pop3
ssl_disable: yes
disable_plaintext_auth: no
login_dir: /var/run/dovecot/login
login_executable(default): /usr/local/libexec/dovecot/imap-login
login_executable(imap): /usr/local/libexec/dovecot/imap-login
login_executable(pop3): /usr/local/libexec/dovecot/pop3-login
mail_max_userip_connections: 100
first_valid_uid: 89
mail_privileged_group: 89
mail_uid: 89
mail_gid: 89
mail_location: maildir:~/Maildir
mail_debug: yes
mmap_disable: yes
mail_nfs_storage: yes
mail_nfs_index: yes
lock_method: dotlock
mail_executable(default): /usr/local/libexec/dovecot/rawlog 
/usr/local/libexec/dovecot/imap
mail_executable(imap): /usr/local/libexec/dovecot/rawlog 
/usr/local/libexec/dovecot/imap
mail_executable(pop3): /usr/local/libexec/dovecot/pop3
mail_plugin_dir(default): /usr/local/lib/dovecot/imap
mail_plugin_dir(imap): /usr/local/lib/dovecot/imap
mail_plugin_dir(pop3): /usr/local/lib/dovecot/pop3
pop3_uidl_format(default): %08Xu%08Xv
pop3_uidl_format(imap): %08Xu%08Xv
pop3_uidl_format(pop3): %f
namespace:
   type: private
   separator: .
   prefix: INBOX.
   inbox: yes
   list: yes
   subscriptions: yes
auth default:
   mechanisms: plain apop
   cache_size: 10240
   cache_negative_ttl: 30
   username_format: %Lu
   passdb:
     driver: sql
     args: /usr/local/etc/dovecot-sql.conf
   userdb:
     driver: sql
     args: /usr/local/etc/dovecot-sql.conf